Globular clusters to hang together for many billions of years. This is because mutual gravitational attraction among a cluster´s hundreds of thousands or even millions of stars keeps stellar members in check.

By looking at the light of their constituent stars, astronomers can measure the ages of globular clusters.

Chemical elements leave signatures in this light. The starlight reveals that stars of globular clusters typically contain fewer heavy elements, such as oxygen, carbon and iron, than stars like the Sun.

Since consecutive generations of stars gradually create these elements through nuclear fusion, stars having fewer of them are relics of earlier epochs in the Universe.

Dating back more than 10 billion years, the stars in globular clusters rank among the oldest on record

There are more than 150 of these objects surrounding our Milky Way Galaxy. On a galactic scale, globular clusters are not all that big. In the case of Messier 68, its stars cover a amount of space with a diameter of little more than a hundred light-years. On the other hand, the disc of the Milky Way extends over some 100 000 light-years or more.